Sulfur-containing flue gases are obtained in copper smelting and are subjected to a flue gas treatment known per se in which the sulfur present is converted to sulfuric acid; the impurities present end up being collected in a washing solution containing sulfuric acid, i.e. so-called washing acid, in which the sulfuric acid concentrations can be between 5% and 35%. Accordingly, the washing acid can have a negative pH. Apart from copper, such a washing acid contains other (heavy) metals, like zinc, cadmium, molybdenum, lead and mercury, as well as other impurities, like arsenic in particular.
Arsenic is an environmental toxin, so one is constantly striving to treat residual or waste materials that arise, such as said washing acids, to free them as far as possible of arsenic and its compounds. It is known e.g. to precipitate arsenic from washing acids as sulfide for this purpose.
DE 34 18 241 A1, for example, discloses a method for removing arsenic from waste sulfuric acids, wherein an aqueous solution of sodium sulfide, NaS2, and sodium hydrogen sulfide, NaHS, in which the amount of sodium sulfide is in stoichiometric excess relative to the arsenic content of the waste sulfuric acid, is used in a hydrogen sulfide atmosphere as a sulfidizing agent.
In such precipitation reactions, copper present in the washing acid is also precipitated as sulfide, which is then disposed of together with the arsenic sulfide and is thus lost.
In the review article “Arsenic—a Review. Part II: Oxidation of Arsenic and ist Removal in Water Treatment”, Monique Bissen et al., Acta hydrochim. hydrobiol. 31 (2003) 2, 97-107, the authors give detailed attention to the oxidation of arsenic and its removal in water treatment, dealing not with sulfide precipitation but with other precipitation reactions, e.g. treatment with FeCl3 at pH 7. However, the article states that As(III) is generally more difficult to remove than As(V) because As(V) precipitates more rapidly than As(III).